I doubt it was a case of running out of time for GT5 as I think you can't upgrade a Standard model into a Premium or downgrade a Premium model since they both require different modelling techniques.
They somehow chose to model it using the old technique as it clearly is a Standard car, not a Premium body without interior like the GT-R Proto.
If they had build it as a Premium and run out of time it perhaps simply wouldn't be included, a half build Premium model is just a half build Premium model, not a texture based Standard car.
As to why, we can only speculate as it sure puzzles me why they included a 2010 model in this way, maybe they'd already build it for GT-PSP in 2009 as one of the last cars (perhaps having early access to that car in the same way they had early access to the BMW 330i in GT4 before it was released to the public) and it indeed was unfinished for GT-PSP or Jaguar asked for it not to be included after all (maybe due to the real car not being released yet) and it ended up on the shelves until it made its first appearance as a Standard car.
SP.
It was understood on the Veyron and R8 LMS they were from GTPSP, but this is a new car that was modeled using GT4 style techniques. It looks like crap!
